The present invention relates to a Machining Center for machining a workpiece by of one out of at least two, automatically interchangeable, tools that are each clamped in an individually driven work spindle and can be applied alternately to the workpiece by numerically controlled movements.
When machining workpieces, the cycle time required for completion comprises machine time, during which the workpiece is physically modified, and also downtime required for all non-productive operations, such as changing workpieces or changing tools on the machine.
For small components with machining cycles requiting different tools, downtime takes up an important fraction of the total cycle time. Due to the short machine time typical for such components and the relatively long chip-to-chip time due to changing tools, downtime can even exceed machine time. In such cases machining of the workpieces does not represent an economic optimum.
Devices are known with spindles applied alternately that aim also at reducing chip-to chip time. Such a device applicable to existing machine tools is described, for example, in the registered design DEV-9015333. It comprises two work spindles, a drive- and a positioning-motor and groups of pinions and clutches. This device permits the changing of tools simultaneously with machine time but still does not resolve the problem of the long chip-to-chip time, as the individual steps of operation, e.g. "unlock indexing", "brake spindle", "actuate clutch", "accelerate spindle" and "unlock indexing" still are sequential in time. No time is thus gained as compared with rapid conventional solutions.
Also similar machines with two individually driven work spindles are known. They are fitted to a common spindle support in opposite directions by 180.degree.. The changing operation is effected by rotating the spindle support by 180.degree.. An example for this is the metal-cutting machine sold under the trademark MONOPOSTO of the CMS OPTICAL company, that makes it possible to carry out movements in parallel operation. However, the relatively long half-circle described during rotation for changing as well as the need for locking and unlocking operations due to the not numerically controlled changing process do not permit appreciable savings in time as compared with rapid conventional solutions.
In addition devices or machines are known such as the ones mentioned above, in which the attempt is made to compensate for the unfavorable influence of the time required for the tool change on the cycle time used to complete an individual workpiece by applying simultaneously several tools to several workpieces. For example, DE 4020997A1 and CH-654237 show machine tools with several spindles for interchangeable tools, the spindles with axes parallel to each other being fitted to a common spindle support. In these machines each spindle machines one particular workpiece with a single tool. For changing tools, the entire spindle support is moved to a changing position. During the time required for this operation (displacement of the spindle support to the changing position, changing of tools, return of the spindle support to the working position) no workpiece can be machined thereby. The unfavorable ratio for machine time to downtime in such a two-spindle machine can only be improved by using several parallel spindles (actually up to eight spindles are used). A very serious drawback of this solution is that, when changing workpieces adjustment work required for alignment of the workpieces is unproportionally time consuming, thus reducing overall productivity and flexibility.
CH-666432 shows a machine tool with two spindles with parallel axes for the workpieces and with two turrets with parallel axes for various tools. The same workpiece can either be machined simultaneously with two tools or in sequence by one single tool in each of the two turrets whereby, collision of file tools or of the turrets is avoided. Automatic changing of tools stored in a magazine is not provided however, in any event, this could only be performed by using the tools previously fitted to each single turret. Thus the improved ratio of machine time to downtime is again deteriorated by the loss in overall productivity and flexibility of the machine. Also the choice of available tools is rather limited.
An object of the present invention is to provide a machining center of the kind mentioned above in such a way as to improve the ratio of machine time to downtime, as compared with the state of the art, without loss of flexibility of the machine tool.
This object has been is achieved in a machining center built according to the present invention by engaging one single spindle out of at least two individually driven spindles, while simultaneously the spindle not engaged is automatically fitted with a new tool in a manner already known. All other movements that determine the chip-to-chip time are performed parallel in time to the movements of the other spindle. These movements are "retract tool from workpiece", "brake down spindle", "accelerate spindle" and "approach tool to workpiece".
In addition, all movements required for the changing of spindles are entirely numerically controlled, thus making any additional locking unnecessary. The ratio of machine time to downtime due to the tool change becomes thus considerably more favorable than known at the present state of the art. At the best, the chip-to-chip time can thus be reduced to the value of the feed time of the working spindle or of the workpiece. Technical realization of the present invention is based on alternate and simultaneous travel to various positions of tool change and possibly of chip-making along numerically controlled axes. Chip-to-chip time depends, therefore, on the attainable speed and acceleration of an NC-axis in rapid traverse.
In the machining center according to the present invention all spindle axes are preferably parallel to each other and linear. As a result, tool change is advantageously performed for all tools similarly and in the same direction, and a minimal number of axes is required for complete translatory movement in space between tool and workpiece. Three linear axes are sufficient. Any additional axis is not required for the actual cutting except for the tool change process or for the protection of a spindle.
In order to obtain the advantage of being able to engage the workpiece with the tool at low expenditure in four, or five directions, the worktable is made to rotate about one or two rotation axes, these axes of rotation together with the axis of the spindle constituting a two or three-dimensional basis respectively.
In one of the preferred configurations of the machining center, the spindles with parallel axes are stationary in space and the worktable moves about three axes that are orthogonal to each other, one of them being parallel to the spindle axes. In this configuration, the worktable is supported over an outrigger by the nearest of the three slides in such a manner that its longitudinal axis is at fight angles to the axes of the spindles and possibly to the connecting line of the parallel spindles
In another preferred configuration of the machining center, the spindles with parallel axes can be moved in an equally parallel direction. Two linear axes for the worktable movement consitute together with the linear axes of movement of the spindles an orthogonal three-dimensional basis. Here, taking of a tool from and returning it to a magazine can be effected preferably by an additional movement of the magazine without a changer arm.
In again another preferred configuration of the machining center, the spindles with parallel axes are placed in spindle supports that can be moved in planes that are also parallel to each other. A single installed linear axis for the worktable movement constitutes together with the axes of the spindle support planes a three dimensional basis. The taking of the tool from and returning it to a magazine is in this case preferably effected without changer arm and without magazine movement.
All preferred configurations are fitted, as described above, with a worktable, rotatable about respectively one or two axes in order to permit presentation of the workpiece at small expenditure to the tool in four or five directions.